Great points, Claude, and thanks for including the hand-drawn illustrations.

Hmmm....Is it possible/fair to say, that the 'virtual pivot point' from the trailer's perspective might actually be an infinite distance forward from the ball, since the trailer cannot induce any swaying/lateral movement on the tow vehicle? Then, the 'apparent pivot point' would only be from the tow vehicle's perspective, and would be about 47" forward of the trailer ball (when in a straight line)? This would seem very consistent with the reality of how this hitch works.

On another note, I wanted to clarify one thing, about what Claude discussed:

Claude B wrote:This is also why with the HA, the TV has always the control of the TT. With this hitch, a jacknife situation is totally impossible in panic stop whatever the speed.

Thats true - with a Hensley, the TV always has control of the TT, as long as the tow vehicle is 'pulling' the TT, and the opposite does not occur (TT 'pushing' the tow vehicle real hard).

However, if during a panic stop, the trailer 'pushes' the tow vehicle too hard, it actually IS possible, for a jackknife to occur with a Hensley. During a panic stop, if the brake controller is not set aggressive enough, and the trailer 'pushes' the tow vehicle hard enough, the Hensley will let the trailer shift off to one side, resulting in a push slightly off to one side. This 'push' affect of the Hensley has been discussed, and is well known among Hensley owners. It just means that with a Hensley (like any other combination/hitch), you must have your brake controller adjusted right, to prevent too much trailer 'push' from occuring during a panic stop.

I will tell you, I once did not have our brake controller set right, and felt this 'push'. I was braking REALLY hard when it happened, and the push was not anything to worry about. Only way I can see it would cause a problem, is if it was a case of a VERY light/small tow vehicle towing a huge trailer, that it should not be towing in the first place, Hensley or not (like some of the combinations CanAm RV puts together..LOL).

Kind of goes back to what you said, Claude - even though the Hensley is such a perfect design, it should not be used as a 'cure-all' for a bad setup. You still need to set everything else up correctly, especially your brake controller. And, you still need to match the tow vehicle to the trailer appropriately.

Perhaps the best way to understand what is going on would be to put together a scale model and do a little bit of 'experimenting' with it. The model only needs to be 2 dimensional. You could do this easily enough with a few small sticks and some glue (and a hitch and rig to measure).

Ron has the best analysis of this I have seen so far. But the other talk about 'eliminating sway' and other absolutes should raise the skeptic meter. The hyperbole should always be a warning that something is missing.

willald wrote:...For starters: HERE is a basic illustration, showing the trapezoid/4 bar linkage, that the Hensley uses to eliminate sway. Some of the details of this description have been accused of being inaccurate, but this still provides a good starting point. I have actually measured our Hensley head, and found that the two side bars are about 4.5" long, the rear (longer) bar on the trailer side is about 8", and the front bar is about 7.25" long...

I sketched the geometry up in SolidWorks and plugged in the numbers you provided for the link lengths. The furthest forward the "virtual" pivot point (if we're using VPP to describe the projected intersection of the two side links) is of course when there is no angle between the TV and the TT. Using your numbers, that point is 43.35 inches in front of the "front bar" (the bar that makes a T with the TV).
Of course, this is using Will's numbers which I believe are only guestimates. We'd have to know the actuals to determine how far up the VPP really is.

bryanl wrote:Perhaps the best way to understand what is going on would be to put together a scale model and do a little bit of 'experimenting' with it. The model only needs to be 2 dimensional. You could do this easily enough with a few small sticks and some glue (and a hitch and rig to measure).

I was actually thinking the same thing, except going to a bit more detail. Perhaps if you could find/build a scale model of a good size TT, and a truck/tow vehicle, then hitch them up with something similar to the 4 bar linkage. You could make the linkage with just 4 sticks like you mentioned here. Then, you could do things like 'push' on the side of the trailer to simulate crosswinds, etc, and see how any pivoting it does, affects the truck. My bet is you'd find that the whole combination would be pushed to the side as one unit, and that the trailer could NOT induce any pivoting. That would answer the question of where the pivot point is, from the trailer's perspective.

Of course, who has the time to do all that? lol!

Quote:Ron has the best analysis of this I have seen so far. But the other talk about 'eliminating sway' and other absolutes should raise the skeptic meter. The hyperbole should always be a warning that something is missing.

I agree, Ron has a great analysis. And, I also agree that something is missing - from Ron's analysis. His analysis does a great job of showing where the pivot point is from the tow vehicle's perspective, but I can't help but wonder if from the trailer's perspective, that 'virtual pivot point' may be at a different place.

willald wrote:...Hmmm....Is it possible/fair to say, that the 'virtual pivot point' from the trailer's perspective might actually be an infinite distance forward from the ball, since the trailer cannot induce any swaying/lateral movement on the tow vehicle?...

No, the VPP could not be infinitely forward from the ball or you'd run into a situation where lateral forces could get leverage on the front of the TV. The only place where you have NO twisting leverage on the TV due to lateral forces is exactly halfway between the rear axle and the front axle. However, anywhere between or close to the axles (probably within 12" on a typical wheelbase TV) will probably make leverage due to lateral forces almost nil.

bryanl wrote:Perhaps the best way to understand what is going on would be to put together a scale model and do a little bit of 'experimenting' with it. The model only needs to be 2 dimensional. You could do this easily enough with a few small sticks and some glue (and a hitch and rig to measure).

I was actually thinking the same thing, except going to a bit more detail. Perhaps if you could find/build a scale model of a good size TT, and a truck/tow vehicle, then hitch them up with something similar to the 4 bar linkage. You could make the linkage with just 4 sticks like you mentioned here. Then, you could do things like 'push' on the side of the trailer to simulate crosswinds, etc, and see how any pivoting it does, affects the truck. My bet is you'd find that the whole combination would be pushed to the side as one unit, and that the trailer could NOT induce any pivoting. That would answer the question of where the pivot point is, from the trailer's perspective.

I have the Hensley video where they use an actual hitch for demonstration purposes. They show that when the TV tries to create an angle with the TT there is no resistance and the angle is easily created. Then they demonstrate that applying strong lateral forces to the TT side of the hitch results in no angle being created - Yes, the combo moves as a single unit. I don't know that that answers where the pivot point is so well, but it sure addresses the concern with TT induced sway.

By the way, I'm not a Hensley owner - just a mechanical engineer who is impressed with how cleverly the Hensley addresses trailer-swinging induced sway. For the price, you'd think they'd be able to do it with a little less added weight by using more titanium parts.

This is a very interesting discussion and I am beginning to understand more cleraly why the Hensley is often referred to as a "sway prevention" hitch instead of a "sway control" hitch. If I understand correctly, when driving in a straight line, the hitch essentially locks the trailer in column behind the TV as if they were a single, non-articulated unit. (I think the Dual Cam sway control cams provide some of this locking action, though from an entirely different approach.) When the TV and TT move out of column, for whatever reason, the hitch geometry projects the point of articulation forward toward the rear axle, in the manner of a fifth wheel, reducing or eliminating the tendancy of the two units to get out of phase with each other. Very clever.

My hitch, which is a straight forward "sway control" hitch simply dampens, but does not prevent, oscillations before they can become a problem. I works very well, but admire the genius of the Hensley hitch. I wish they were cheaper.

willald wrote:...His analysis does a great job of showing where the pivot point is from the tow vehicle's perspective, but I can't help but wonder if from the trailer's perspective, that 'virtual pivot point' may be at a different place.

If we really went crazy, we could break it down to free body diagrams and increase the scope from simple theoretical points to analyzing specific forces, points of application, centers of mass, moments of inertia, etc. Personally, I'd rather just look at the 4-link hitch as a sophisticated hinge that allows twisting forces to create an angle and prevents lateral forces at the "hinge" from doing the same.

Tim, I believe you are right, in that free body diagrams is the only accurate way to look at it. But, I think in the end, the VPP will be the intersection of the side bars of the 4 bar linkage as you diagram above. That pivot point will move as this linkage flexes. Is it correct that two of the bars are effectively rigidly attached to the tow vehicle and the other to the trailer?